Driving safety and driving economy play an important role in automobile design. Tire pressure monitoring systems, which contribute to safe driving and enhanced fuel efficiency, are increasingly integrated in automobile equipment. Two different techniques, namely direct and indirect tire pressure measurements, are distinguished in the prior art. Direct measurements, which are very precise, require comparatively expensive equipment integrated within the tire. Indirect measurement systems, which may be based on anti-locking systems (ABS), are easily implemented and cost-efficient but up to now less precise than direct measurement techniques.
Indirect measurement techniques comprise wheel radius analysis (WRA) and wheel vibration analysis (WVA).
In wheel radius analysis, the wheel radii are estimated by comparing the wheel speeds of each vehicle wheel (which may be measured by wheel speed sensors of the ABS) with the vehicle's actual driving velocity. Constant driving velocity provided, the wheel speed increases when the wheel radius decreases due to tire pressure loss.
In wheel vibration analysis, wheel vibration phenomena (which may be measured by wheel speed sensors of the ABS) are detected within the time dependent behavior of the rotational velocity of an individual vehicle wheel. Tires in contact with the road surface during driving are exposed to vibrations. Here, the fact is exploited that the spectral properties (e.g. the energy distribution in different frequency bands) of a wheel angular velocity signal depends on the tire pressure of the individual vehicle wheel.
Traditional tire pressure estimating systems rely on either wheel radius analysis or wheel vibration analysis. Both systems have particular advantages and deficiencies. Estimating tire pressures with wheel radius analysis is easily implemented but affected by vehicle load changes or driving forces during cornering, acceleration and braking, which also lead to wheel radius changes. Wheel vibration analysis, which requires increased computing performance, is very precise in a low and medium velocity range, but shows deficiencies in a high velocity range of the driving velocity.
JP 5-133831 discloses a WVA which identifies a pronounced low frequency resonance which can be measured when the vehicle velocity is low. Typically, the corresponding resonance frequency lies in the range of about 30 to 50 Hz. When the vehicle velocity increases to a high velocity range, the low resonance peak looses intensity. In this velocity range the JP 7-21723 and EP 0925960 further identify a high frequency resonance which is suitable for the tire pressure estimation. The frequency of this resonance is in the range of about 60 to 90 Hz. However, for too high vehicle velocities, it becomes increasingly difficult to use vibrational analysis for tire pressure estimation. FIGS. 12A and 12B show two power spectra of the rotational velocities which correspond to a low and a high vehicle speed, respectively. The low (FIG. 12A) and high (FIG. 12B) resonance peaks can easily be identified.
The low resonance peak is generally ascribed to a torsional resonance in the rotational direction of a tire. When the tire air pressure drops the spring constant in the torsional direction decreases. The high resonance peak is either ascribed to secondary components of the torsional resonance frequency or to tread pattern effects. FIG. 13 shows a typical relation of the tire pressure as a function of the resonance frequency (high resonance peak).
EP 0 925 960 and WO 01/87647 disclose tire air pressure estimating systems implementing both analysis techniques, WVA and WRA, to compensate for the individual deficiencies of these different analysis techniques. Hereby, the system described in EP 0 925 960 switches between the two analysis techniques depending on the current driving parameters and conditions (vehicle velocity, resonance signal intensity, vibration input intensity, brake switch signal, etc.). When the vehicle is in a low or medium velocity range, where WVA is efficient, the tire air pressure estimating is based on the output of the WVA. In the high velocity range, where WVA is less efficient, it relies on the output of the WRA. The tire pressure estimating system disclosed in WO 01/87647 derives for each vehicle wheel two individual confidence level values corresponding to the outputs of the two analysis techniques, WVA and WRA, respectively. If the outputs of the WVA and the WRA correspond to each other, then the sum of the corresponding two individual confidence values is compared with a first threshold value. Otherwise, the higher value among the two individual confidence level values is chosen and compared with a corresponding threshold value (one threshold value for the WVA and WRA, respectively). A tire pressure indication signal indicating a tire pressure loss is generated when one of the three threshold conditions is fulfilled.